* TSX down 168.77 points, or 1.25 percent, to 13,308.77
* Eight of the TSX's 10 main groups are lower
TORONTO, March 15 (Reuters) - Canada's main stock index fell on Tuesday as weaker commodity prices weighed on energy and mining stocks, while the shares of a major drugmaker tanked.
Valeant Pharmaceuticals International Inc plunged more than 40 percent to C$50.84 after it cut its 2016 revenue forecast by about 12 percent and said a delay in filing its annual report could pose a debt default risk.
Energy sector shares retreated 2.2 percent, while the materials group, which includes precious and base metals miners and fertilizer companies, lost 0.9 percent.
Oil and gas producer Canadian Natural Resources Ltd fell 3.4 percent to C$34.88, while Cenovus Energy Inc was down 2.0 percent at C$16.99.
U.S. crude prices fell for a second day, down 2.7 percent at $36.18 a barrel, as concerns emerged that a six-week rally may have fizzled after OPEC doused hopes for a speedy erosion of a global overhang of unwanted crude.
Gold fell to its lowest in almost two weeks ahead of a Federal Reserve meeting that is expected to give clues on the pace of future U.S. rate rises, while copper also dipped.
Among Canadian mining stocks, First Quantum Minerals Ltd , declined 7 percent to C$7.44, while Teck Resources Ltd fell 7.3 percent to C$9.23.
At 11:12 a.m. EST (1512 GMT), the Toronto Stock Exchange's S&P/TSX composite index fell 168.77 points, or 1.25 percent, to 13,308.77. On Friday, the index posted a three-month high at 13,543.29.
Eight of the index's 10 main groups were lower.
The financials group fell 0.3 percent, including a 1.3 percent drop in Manulife Financial Corp to C$18.51. (Reporting by Fergal Smith Editing by W Simon)